1. Field of the Invention
The present invention relates to methods for producing a composite comprising plural metal oxides, a composite comprising a metal oxide and microparticles of a material other than the metal oxide, and a composite of metal oxide doped with metal ions. In particular, the production methods of the present invention can be applied even to materials of which substrates are not heat-resistant since heating at a high temperature is not required. The composites and oxides doped with metal ions obtained by the methods of the present invention are useful as, depending the nature of the oxides, materials for photocatalysts, memory devices, batteries, sensors, photonics and the like.
The present invention further relates to a method for forming a titanium oxide coating layer on plastic articles. In particular, because the method of the present invention does not require heating at a high temperature, it enables formation of a titanium oxide coating layer on heat-labile plastic substrates, which had been difficult by conventional methods. The photocatalytic function of titanium oxide can be imparted to plastic articles by the methods of the present invention.
The present invention also relates to a method for producing titanium oxide doped with metal ions, which is useful as a photocatalyst having visible light absorbing properties, and a photocatalyst having visible light absorbing properties.
The present invention further relates to a titanium oxide based anti-fogging material, a coating film comprising the anti-fogging material, and a substrate having the coating film such as spectacle lenses. The anti-fogging material of the present invention has excellent anti-fogging properties, and is a anti-fogging material of a low surface reflectance. The present invention further relates to a production method of the aforementioned anti-fogging material.
2. Background Art
It has been known that titanium oxide has photocatalytic function, and its applications are being developed in various fields because antibacterial, antifouling, and anti-fogging properties can be imparted by forming titanium oxide layers on surfaces.
However, titanium oxide can utilize light with limited wavelength in ultraviolet range. In order to utilize light contained in sun light effectively, it is desirable to utilize not only light in ultraviolet range but also light in visible light range. In this respect, it has been known that titanium oxide originally exhibiting catalytic reactions only with UV absorption is converted to one utilizable light in visible light range by doping with chromium ions (see, for example, Kubokawa, Honda, and Saito, xe2x80x9cPhotocatalystxe2x80x9d, (1998)). In particular, Anpo et al. have successfully formed a titanium oxide film of visible light absorbing type by doping titanium oxide with chromium ions without aggregation of the ions through injection of the chromium ions by ion-implantation technique. Anpo et al. have succeeded in decomposition of NOx with light at 450 nm, which is within the visible light range, by using titanium oxide injected with chromium ions through the ion-implantation technique (1996, Proc. In DOE Workshop on Solar Hydrogen Production (1996)). In general, doping of semiconductor materials with a certain metal causes an impurity level in the semiconductor materials, thereby a new absorption band is added to absorption bands of the materials. However, it has been considered that because electrons and positive holes formed by light excitation of band gap are recombined on the chromium ions and inactivated, the photocatalytic reaction would not proceed. To the contrary, Anpo et al. have made it possible to control physical properties of solid titanium oxide by injecting chromium ions thereinto through irradiation of metal ion beam accelerated to high energy.
As a typical example of the technique for ceramic thin film formation under a low temperature condition which is utilizable in the field of photocatalysts, the fog-resistant coating compositions can be mentioned (Japanese Patent Unexamined Publication No. Hei 9-59041/1997). These compositions comprises a mixture consisting of a blend of a silicon precursor, amorphous silica precursor and photocatalyst particles (specifically, a mixture of silica sol, trimethoxymethylsilane, and titania sol), and thin films are made by applying the mixture on a substrate, and sintering it at a low temperature, i.e., 150xc2x0 C.
By the Kanagawa Industrial Technology Research Institute, gold-titania composite ultramicroparticles having a core/shell structure have been produced by surrounding gold particles as nuclei with titanium oxide by the vapor-in-gas technique. Applications of these composite ultramicroparticles as materials of catalysts, wet type solar batteries, and capacitors are studied because they exhibit sun light absorbing properties and thermal stability.
As described above, it has been attempted to add a third component to a metal oxide or make a composite of metal oxides in order to improve function of the metal oxide or to prolong the duration where the function is retained. However, the aforementioned ion-implantation method requires enormous facility and hence is impractical, and the vapor method requires vaporization of the starting metal oxide material at a high temperature and has problems as described hereinafter.
It has also been known to use platinum and ruthenium oxide as a promoter along with titanium oxide when titanium oxide is used as a photocatalyst. It is preferred that these promoters should be incorporated into the surface of titanium oxide as dense as possible from the viewpoint of improvement of the catalytic function.
However, such a process requiring sintering at a high temperature for making a composite with a third component to a catalyst may not yield a desired composite, because it may form an intermediate according to chemical thermodynamics depending on the nature of the material to be combined.
Therefore, the first object of the present invention is to provide a method for making a composite of a metal oxide with another oxide or metal as a third component, or doping a metal oxide with metal ions as a third component without treatment at a high temperature.
As described above, there has been developed a technique capable of forming a titanium oxide coating film with sintering at a lower temperature (100-150xc2x0 C.) (Japanese Patent Unexamined Publication No. Hei 9-59041/1997). However, when the articles desired to be imparted with the photocatalytic function are poorly heat-resistant plastic materials or the like, the aforementioned method for forming coating films using a sintering step is not applicable.
Therefore, a titanium oxide coating material of low temperature drying type has been developed. This material is for adhering titanium oxide particles to substrates with an organic or inorganic binder. With this coating material, films are formed by spray coating, dip coating, photogravure printing and the like as ordinary paints. While the obtained films will have practical transparency, peeling property and weather resistance, they disadvantageously have a low hardness of 3-5H. It is obviously inferior than that of the films obtained by sintering at high temperature, which is 5-7H. From the viewpoint of practical use, such hardness as mentioned above may be a significant drawback in a certain field requiring scratch resistance.
Therefore, the second object of the present invention is to provide a method capable of forming titanium oxide coating layers having transparency, low peeling property and weather resistance, which are comparable to or even more excellent than those of conventional ones, and surface hardness comparable to that obtained by the sintering method without requiring any treatment at a high temperature.
The titanium oxide injected with chromium ions, which is produced by the aforementioned method of Anpo et al., may show photocatalytic activity even with light within visible light range. Although the ion-implantation technique is a common technique in the semiconductor industry, it requires vast equipment and high operation cost, and hence it is difficult to use it for ordinary production of photocatalysts.
Therefore, the third object of the present invention is to provide a method for producing titanium oxide showing photocatalytic activity even with light within visible light range with a markedly simpler and easier process compared with the ion-implantation method. The fourth object of the present invention is to provide a photocatalyst showing photocatalytic activity even with light within visible light range, which is produced by the method mentioned above.
In order to obviate the high reflectance of titanium oxide coating layers, a material showing low index of refraction such as silica is conventionally admixed in them. This is because titanium oxide has a high index of refraction of 2.60, and when it is provided on a substrate of a low index of refraction such as glass substrates, the titanium oxide layer serves as a reflecting layer to afford a high reflectance, which causes problems that, for example, when coated on spectacle lenses, they may reduce visual field and deteriorate the appearance of the lenses. As such thin films, there may be mentioned the aforementioned titanium oxide/silica mixed thin films disclosed in Japanese Patent Unexamined Publication No. Hei 9-59041/1997. According to this patent document, coating films are formed by sintering a coating film formed through the sol-gel process on a substrate at a low temperature (100-150xc2x0 C.).
However, the photocatalytic function of titanium oxide is diluted and reduced by the addition of silica to titanium oxide. In addition, the aforementioned method requires sintering at 100-150xc2x0 C. Though this may be considered a low temperature range, it is impossible to provide layers on a substrate of low heat resistance such as plastic substrates by the above method. For example, it is desirable to impart anti-fogging properties to a low heat resistance material such as mirrors, spectacle lenses and the like mad of plastic materials by forming on it films having high and durable anti-fogging properties.
Therefore, the fifth object of the present invention is to provide a anti-fogging material having transparency, peeling property and weather resistance, which are comparable to or even more excellent than those of conventional ones, surface hardness comparable to that of coating layers obtained by the sintering method, reflectance reduced by decreasing the difference of index of refraction from that of substrate, and durable anti-fogging properties, and not requiring a treatment at high temperature. The sixth object of the present invention is to provide a substrate having the aforementioned anti-fogging material as a coating layer, and a method for producing the aforementioned anti-fogging material which does not require a treatment at a high temperature.
An aspect of the present invention, which achieves the first object mentioned above, relates to a method for producing a composite composed of a mixture containing a metal oxide derived from a metal fluoro complex compound, which comprises adding a fluoride ion-capturing agent to an aqueous solution containing the metal fluoro complex compound to precipitate the composite (referred to as the method for producing composites of the present invention hereinafter).
In the method for producing composites of the present invention, the aqueous solution containing a metal fluoro complex compound may be an aqueous solution containing two or more kinds of metal fluoro complex compounds; and the composite may be precipitated in the presence of two or more kinds of seed crystals comprising metal oxides the same as those derived from the two or more kinds of metal fluoro complex compounds to form a composite comprising a mixture of two or more kinds of metal oxides (these metal oxides are both in a stable phase) derived from the metal fluoro complex compounds.
In the method for producing composites of the present invention, the aqueous solution containing a metal fluoro complex compound may contain microparticles, and the composite to be formed may be a mixture of a metal oxide derived from the metal fluoro complex compound and the microparticles; or the aqueous solution containing a metal fluoro complex compound may contain a water-soluble metal compound; and the composite to be formed may be a metal oxide which is derived from the metal fluoro complex compound and doped with metal ions derived from the water-soluble metal compound; or the aqueous solution containing a metal fluoro complex compound may contain a seed crystal comprising metal oxide which is the same as that formed from the metal fluoro complex compound.
In the method for producing composites of the present invention, the composite may be in the form of a thin film precipitated on a substrate immersed in the aqueous solution containing a metal fluoro complex compound.
Another aspect of the present invention, which achieves the second object mentioned above, relates to a method for forming a titanium oxide coating layer on a plastic substrate, which comprises adding a fluoride ion-capturing agent to an aqueous solution containing a fluorotitanium complex compound to precipitate a titanium oxide coating layer on a plastic substrate immersed in the aqueous solution (referred to as the method for precipitating titanium oxide coating layers of the present invention hereinafter).
In the method for precipitating titanium oxide coating layers of the present invention, the aqueous solution containing a fluorotitanium complex compound may contain titanium oxide particles. In the method for precipitating titanium oxide coating layers of the present invention, the plastic substrate may have a underlying layer on its surface on which the coating layer is formed. The underlying layer may be an oxide layer or fluoride layer, and the oxide layer may be a silicon oxide layer.
In the method for precipitating titanium oxide coating layers of the present invention, the aqueous solution containing a fluorotitanium complex compound may contain at least one kind of material selected from the group consisting of metal oxide colloidal particles, metal colloidal particles, organic material particles and water-soluble metal compound, and the titanium oxide coating layer may be a layer comprising a composite of titanium oxide derived from the fluorotitanium complex compound with the particles and/or metal ions derived from the compound.
Another aspect of the present invention, which achieves the aforementioned third object of the present invention, relates to a method for producing visible light absorbable titanium oxide, which comprises adding a fluoride ion-capturing agent to an aqueous solution containing a fluorotitanium complex compound and a metal compound to precipitate titanium oxide doped with metal ions derived from the metal compound (referred to as the method for producing visible light absorbable titanium oxide of the present invention hereinafter).
In the above method for producing visible light absorbable titanium oxide of the present invention, the metal ions may be at least one kind of ions selected from the group consisting of chromium ions, iron ions and vanadium ions. The aqueous solution may contain a seed crystal comprising titanium oxide. The method for producing visible light absorbable titanium oxide of the present invention may be a method comprising recovering particles of titanium oxide doped with metal ions, which particles have been precipitated in the aqueous solution.
In the above method for producing visible light absorbable titanium oxide of the present invention, which achieves the fourth object of the present invention, a thin film of titanium oxide doped with metal ions may be precipitated on a substrate immersed in the aqueous solution. In the method for producing visible light absorbable titanium oxide of the present invention, the precipitated titanium oxide doped with metal ions may be subjected to a heat treatment to obtain a uniform doping level.
The present invention also relates to a visible light absorbable photocatalyst composed of titanium oxide doped with metal ions, which is produced by the aforementioned method for producing visible light absorbable titanium oxide of the present invention (referred to as visible light absorbable photocatalyst of the present invention hereinafter).
In the visible light absorbable photocatalyst of the present invention, the titanium oxide doped with metal ions may be powder or a thin film on a substrate.
Another aspect of the present invention, which achieves the above fifth object of the present invention, relates to a anti-fogging material comprising (1) titanium oxide and a metal oxide having an index of refraction different from that of titanium oxide, or an oxide containing titanium and a metal (metal oxide of the metal has an index of refraction different from that of titanium oxide), (2) noble metal particles, and (3) transition metal (referred to as the anti-fogging material of the present invention hereinafter).
With the aforementioned anti-fogging material of the present invention, for example, a coating film composed of the anti-fogging material of the present invention, or a substrate having the coating film on its surface can be provided. This substrate may be composed of a plastic, and the coating film may be formed on an underlying layer provided on the plastic. The substrate may be a spectacle lens.
Another aspect of the present invention, which achieves the above sixth object of the present invention, relates to a method for producing the aforementioned anti-fogging material of the present invention comprising adding a fluoride ion-capturing agent to an aqueous solution containing at least a fluorotitanium complex compound, metal fluoro complex compound (metal oxide derived from this compound has an index of refraction different from that of titanium oxide), noble metal colloidal particles or noble metal compound, and transition metal compound to form a precipitate (referred to as the method for producing the anti-fogging material of the present invention).